Content data is downloaded from an information processing device such as a content server that holds the content data such as streaming video to a terminal device of a smartphone or the like through a network. In this case, it is demanded to shorten a time demanded to download and to decrease traffic on the network. For these requirements, a technology has been proposed in which a cache server is provided on a base station or a higher network side of the base station wirelessly coupled to the terminal device. According to this technology, the content data transmitted from the content server to the terminal device through the base station is stored (cached) in the cache server. In a case where a request for the same content is subsequently made from the terminal device, the cache server transmits the content data to the terminal device instead of the content server. In this case, the request is not transmitted to the content server. With this, it is possible to shorten the time demanded for the terminal device to acquire the content data after requesting the content from the terminal device. In addition, it is possible to reduce traffic on the network.
In a network including a plurality of cache servers, a related art in which a first cache server transmits the content data held by itself to another second cache server is known. In this related art, in a case where the second cache server receives a content request (content transmission request) from the terminal device, the second cache server obtains the content data from the first cache server, and can transmit the obtained content data to the terminal device instead of the content server. In order to realize this process, the second cache server issues an inquiry about whether or not the first cache server holds the content data which is a target of the request. In a case where the first cache server holds the content data which is the target of the request of the content request, the first cache server transmits target content data to the second cache server. In this specification, there is a case where content which is a request target of the content request is referred to as “target content”. In addition, there is a case where the content data which is the target of the request of the content request is referred to as the “target content data”. As the related arts, there are Japanese Laid-open Patent Publication No. 2006-311013 and Japanese Laid-open Patent Publication No. 2014-35601.